Testing 1, 2… Testing

The COVID-19 pandemic has been a great source of uncertainty for all, with fluctuating and sometimes confusing information about how to stay safe and how we’ll return to normal life. One of the most widely discussed topics has been the need for testing. Testing is important in guiding treatment, limiting disease spread, understanding the related epidemiology, and influencing decisions about public policy. While there’s no doubt about the importance of testing, the different types of tests available and what they mean is less clear. I hope to demystify some of that here.

To better explain the differences in testing, it's helpful to know a bit about how the immune system works.

The role of the immune system is to protect the body against infection and disease. Anything your immune system recognises as ‘foreign invaders’ are called pathogens. Pathogens such as viruses, bacteria, and fungus trigger the immune system to activate specialized white blood cells to work together to fight the pathogen. One way they do this is by producing antibodies that attach to part of the pathogen, called the antigen, and neutralize it. There are different types of antibodies, which do different things. One antibody is called IgM and is made within days of infection but then disappears after several weeks. Another type, known as IgG, typically takes more time to appear but also remains in the body longer.

So what types of testing are available for COVID-19?

There are three broad types of tests:

1. A lab-based test to see if you have the virus, known as the virus PCR test, also sometimes called a molecular or nucleic acid test
2. A lab-based test to see if you have developed antibodies to the virus, known as an antibody test, also sometimes called a serology test
3. A rapid diagnostic test (RDT) or point-of-care test to get quick results without needing to send the sample to a laboratory (different rapid diagnostic tests can be used to see whether the virus or antibodies are present)

Virus PCR Test

The virus PCR test detects the presence of the foreign invader, which in this case is the virus known as SARS-CoV-2. This requires a swab of your nose and throat which can be taken at home, testing centre, or hospital that will then be sent to a laboratory to look for genetic material specific to this virus using a technology called polymerase chain reaction or PCR. This test is useful for identifying whether you are currently infected which is why it is recommended while you are experiencing symptoms. This is the test currently* being made widely available by the NHS either as an at-home sampling kit or at regional drive-through centres.

Antibody Test

The antibody test instead requires a sample of your blood (collected at home with a finger prick test, testing centre or hospital) and then sent to a laboratory to look for the presence of antibodies rather than the virus itself. Because it takes time for your immune system to make these antibodies, this test is best used to identify a past infection. The amount of IgG antibodies produced depends on the severity of the infection¹ and we don’t yet know how long these antibodies will persist in the blood, although studies on a similar virus have shown they last for two to three years². This test may also be useful for those who have been infected but didn’t have symptoms or did not qualify for the virus PCR test. Studies to determine whether SARS-CoV-2 antibodies can protect against future infection are ongoing.

Rapid Diagnostic Tests

The third type of testing that you may have heard of recently is rapid diagnostic tests (RDTs), also known as point-of-care tests. These are usually done in a doctor’s office and give results within minutes without the need to send the swab or blood sample to a laboratory. These tests can look either for virus antigens (rapid antigen test) to help diagnose current infection or antibodies (rapid antibody test) by putting a few drops of blood onto a small cassette to determine a past infection. At present, the World Health Organization recommends these tests only for research purposes but not for clinical decision making until more evidence on their validity and utility becomes available³. Some companies have been illegally selling rapid diagnostic tests for at-home use. As it stands, there are no rapid diagnostic tests approved in the UK for at-home use⁴.

There is still much that is unknown about COVID-19 and doctors and scientists are working hard to discover as much as they can as quickly as possible. Testing both for active infection with virus PCR swab tests, as well as previous infection with antibody blood tests, will play a very important role in our understanding of this disease.

*At the time this article was written, the viral PCR test was the test most widely used by the NHS. This could change with time as antibody testing becomes more available in the future. Up to date information on NHS testing can be found here.

References

1. Long, Q., Liu, B., Deng, H. et al. Antibody responses to SARS-CoV-2 in patients with COVID-19. Nat Med (2020). https://doi.org/10.1038/s41591-020-0897-1
2. Wu, L. P., Wang, N. C., Chang, Y. H., Tian, X. Y., Na, D. Y., Zhang, L. Y., Zheng, L., Lan, T., Wang, L. F., & Liang, G. D. (2007). Duration of antibody responses after severe acute respiratory syndrome. Emerging infectious diseases, 13(10), 1562–1564. https://doi.org/10.3201/eid1310.070576
3. WHO Scientific Brief. Advice on the use of point-of-care immunodiagnostic tests for COVID-19. April 8, 2020.
4. Gov.uk Guidance on coronavirus (COVID-19) tests and testing kits (last updated April 24, 2020 at the time of this publication)